Baxi combi 105e pressure loss?

my baxi 105e is just over 3yrs old. The pressure keeps dropping to nothing and have to repressurise it several times a day. Thought it was a leaking rad causing it so replaced it by using the' pipe freeze' method. Bled all rads after and it seemed ok for about 24hrs. Now the pressure is dropping constantly.. after i top it up and switch the boiler on the pressure drops then gradually rises. Its so bad that i have to get up several times a night to repressurise it as the heating needs to be on constantly due to my 92 yr old gran being in the house. Can anyone help with suggestions??????? PLease